Composite structures are employed in an increasing number of applications, such as a variety of automotive and aviation applications. For example, modern commercial and military aircraft include a number of composite components, such as components fabricated from fiber reinforced composites, to increase the relative stiffness and strength of the components without unnecessarily increasing the weight of the aircraft.
Regardless of the particular application, composite components can be formed by laying up or stacking a number of plies, such as on a tool or mandrel which, at least partially, defines the shape of the resulting composite structure. The plies are thereafter consolidated, such as by an autoclave process or a fiber placement process, into an integral laminate structure. As known to those skilled in the art, the consolidation process typically requires the stacked plies to be exposed to relatively high temperatures and relatively high pressures in order to sufficiently bond the plies into the integral laminate structure. For example, the consolidation of carbon fiber-reinforced plies by a fiber placement process can require temperatures as great as 1300.degree. F. and pressures as great as 600 to 800 PSI.
In addition to conventional autoclave processes, composite components can be fabricated by a fiber placement process in which plies of fibrous tow pre-impregnated with thermoset or thermoplastic resin, typically termed prepregs, are individually placed on and consolidated to an underlying composite structure. Preferably, a laser heats the lower surface of the fiber-placed ply and the upper surface of the underlying composite structure to at least partially melt a localized region of the ply. Compactive pressure is then applied to the at least partially molten region of the ply, such as by a roller disposed downstream of the laser, so as to consolidate the fiber-placed ply and the underlying composite structure, thereby forming the integral laminate structure. One advantage of a fiber placement process is that the composite material can be cured on the fly, thereby reducing the time required to fabricate a composite part.
Another method of fabricating composite components is a resin transfer molding (RTM) process. According to a RTM process, a number of fibers, such as graphite or glass fibers, are woven to form a woven fiber intermediate structure. For example, the fibers can be woven on a loom-type structure as known to those skilled in the art. Resin can then be introduced to the woven fiber intermediate structure such that, once the resin has cured, the resulting composite component formed from the resin-impregnated woven fiber structure is created.
An emerging area of interest with respect to composite structures, regardless of the particular method by which composite components are fabricated, involves the design and development of smart structures. Smart structures generally refers to composite structures which include one or more interactive electronic devices. For example, monolithic or multi-layer electroceramic actuators can be embedded within a composite structure so as to induce vibrations within the composite structure. In particular, an electroceramic actuator can induce vibrations in the composite structure in order to offset or damp externally induced vibrations of the composite structure. In addition, smart structures can include other electrical devices, such as antennas and integrated circuits.
In order to protect the embedded electrical device during the fabrication of the composite structure by an autoclave or fiber placement process, the electrical device is preferably disposed within a trough formed in a ply of composite material. Thereafter, additional composite layers can be stacked on the ply and consolidated thereto to form an integral laminate composite structure in which the electrical device is embedded. Alternatively, the electrical device can be embedded within the woven fiber intermediate structure prior to doping the structure with resin in a RTM process.
Even if the electrical device withstands the fabrication process, including the relatively high temperatures and relatively high pressures to which the device is exposed during consolidation, the electrical device must still be able to receive, and in many instances, transmit signals in order to function as desired. Accordingly, the embedded electrical device, such as an electroceramic actuator, typically includes a pair of electrical leads which are routed to the surface of the resulting composite structure in order to provide for an external electrical connection, such as with an external controller. In addition, numerous composite structures include one or more plies which are comprised of a conductive material. Therefore, the electrical leads of the embedded electrical devices are typically coated with an insulating material, such as Kapton.TM. material, in order to electrically isolate the electrical leads from the conductive plies of the composite structure.
A composite structure generally includes inner and outer surfaces through which the electrical leads of the embedded electrical device extend. In order to facilitate connection with other electrical devices, the electrical leads are typically routed through the inner surface of the resulting composite structure. Accordingly, troughs or bores must be formed or cut in the composite structure, such as from the interior surface thereof, so that the electrical leads can extend therethrough.
However, the surface egress of the electrical leads of an embedded electrical device as described above is primarily effective in instances in which a hollow composite structure is fabricated, such as a trapezoidal rail, which permits the electrical leads to be routed to the hollow interior of the composite structure. In contrast, in instances in which the composite structure is not hollow, such as a solid or a relatively planer composite structure, the surface egress of the electrical leads of the embedded electrical device is less effective since the electrical leads will protrude from a surface, such as the exterior surface, of the composite structure and may interfere with the performance of the structure. For example, electrical leads which protrude through the exterior surface of a composite fairing can be exposed to potentially harmful environmental conditions, can create undesirable wind resistance and can otherwise impair the performance of the composite structure.
Even if the resulting composite structure is hollow, the electrical leads must oftentimes be secured within the respective troughs defined in the composite structure with an adhesive in order to retain the electrical leads within the respective troughs. However, the application of adhesive within the trough can complicate the fabrication process by impairing the placement and consolidation of additional plies over the embedded electrical device.
As described above, composite structures are oftentimes formed by stacking or laying up a plurality of plies on a tool which defines, at least in part, the shape of the resulting composite structure. Accordingly, in order to route the electrical leads from the embedded electrical device through the interior surface of the composite structure, the tool, such as a mandrel, must be hollow so as to define an internal cavity into which the electrical leads must extend. Thus, the trough defined in the composite structure through which the electrical leads extend must be carefully aligned with a corresponding aperture defined by the mandrel such that the electrical leads can extend through the aperture defined in the mandrel and into the internal cavity of the hollow mandrel. In addition, the electrical leads might need to be secured within the respective apertures using specialized tools which may, in turn, further impair the placement and consolidation of additional plies over the egressed lead.
Due to the construction of a conventional breakdown mandrel, the internal cavity of the mandrel cannot generally be accessed during the fabrication of a composite structure. Accordingly, the electrical leads extend into and are disposed within the hollow mandrel in a random order. Thus, the electrical leads can become entangled with other electrical leads or with other surface-egressed components, such as optical fibers, to form a tangled web which is relatively difficult to disentangle. In addition, the electrical leads which extend into the hollow mandrel can sever other surface-egressed components, such as optical fibers, and can render repair of the several components difficult, thereby impairing the performance of the resulting composite structure.